The invention relates to methods and instruments for measuring daughter-ion spectra in reflector time-of-flight mass spectrometers with post-acceleration of parent and daughter ions selected by means of a parent-ion selector.
The mass-to-charge ratio m/z of ions can be determined from their flight time in a time-of-flight mass spectrometer. For the sake of simplicity, only the mass m and its determination will be referred to in the following even though the measurement of the mass to charge ratio m/z, where z is the number of elementary charges carried by the ion, is always used in mass spectrometry. Since many types of ionization, such as Matrix-Assisted Laser Desorption and Ionization (MALDI), in the main produce only singly charged ions (z=1), the differences between m/z and m cease to be relevant for these forms of ionization.
The daughter ion or fragment ion spectra of parent ions which are selected by an ion selector on the basis of their time of flight can be measured in a time-of-flight mass spectrometer which has been fitted with an ion selector and a velocity-focusing reflector. The decomposition of parent ions into daughter ions or fragment ions can be produced by two different processes: firstly, by introducing excess energy while they are being ionized during laser bombardment (xe2x80x98LIDxe2x80x99 Laser Induced Decomposition), where so-called xe2x80x98metastablexe2x80x99 ions are produced which partially decompose as they travel through the mass spectrometer and secondly, by Collisionally Induced Decomposition (CID), which essentially leads to spontaneous decomposition, where somewhat different fragmentation rules prevail, such as the loss of side chains.
The reflector which has now gained general acceptance is the Mamyrin velocity-focusing, two stage reflector. In the first deceleration stage of the reflector, the ions are decelerated sharply, but in the second stage only weakly. Faster ions with the same mass as slower ions penetrate further into the relatively weak linear deceleration field of the second stage and therefore cover a somewhat longer distance which, by correct adjustment of the two deceleration fields, can compensate for the faster speed of the ions with the same mass emerging from a primary focus so that they arrive at the secondary focus at exactly the same time.
As well as the velocity focusing of ions with the same mass, there is energy dispersion for ions with the same velocity but with different masses. If the parent ions and the daughter ions which arise from the decomposition of parent ions enter the reflector simultaneously and with the same velocities, and therefore with different mass-dependent energy levels, they will be dispersed in the reflector by their different energies in accordance with their masses. This dispersion can be used for measuring the daughter ions.
However, the method of detecting daughter ions or fragment ions using reflectors such as these has serious disadvantages. When focusing is reasonably good, only ions with a relatively low relative energy range can be detectedxe2x80x94in the standard commercially available instruments, approximately 25-30% of the energy (and mass) range under adjustment. Thus, for a medium-sized peptide, approximately 10-15 spectral segments have to be acquired if the entire fragment spectrum of the low masses of individual ionized amino acids up to the mass of the parent ions is to be measured. All these spectral segments have to be co-ordinated with each other using a complex mass-calibration process. Only then can the spectral segments be assembled in a data system to produce an artificially generated composite spectrum.
In the patent DE 198 56 014 C2 (U.S. Pat. No. 6,300,627), ways are described for recording daughter-ion spectra in a time-of-flight mass spectrometer with a two-stage reflector using a single non-segmented spectral scan. The patent also discloses other information regarding the MALDI ionization method and velocity focusing by delayed acceleration in the ion source. This method not only saves time when scanning the spectrum but also requires lower sample consumption, and this means that the sensitivity is significantly higher.
The method described in patent DE 198 56 014 C2 is based on the fact that the selected ions are post-accelerated so that they all fit within the energy window of the reflector. One of the methods suggested consists of only slightly accelerating the ions in an ion source with delayed acceleration, allowing them to decompose in an initial drift region, very rapidly raising them to a second acceleration potential using a potential cell (xe2x80x98potential liftxe2x80x99) and accelerating them in subsequent acceleration sections to a second drift region. The second drift region can be at the same potential as the first drift region. In the preferred embodiment, the two drift regions are operated at ground potential. In the second drift region, very light ions then have a minimum energy which provides the second acceleration potential; the parent ions which have not decomposed have a maximum energy corresponding to the sum of the first and second acceleration. If a reflector is able to reflect particles with energy deviations corresponding to approximately 30% of the maximum energy, and if about 70% of the total energy is provided by the second acceleration potential, then the reflector can reflect all the daughter ions simultaneously. In this case scanning of the entire daughter-ion spectrum without any segmentation is possible.
At the same time, the potential lift itself can be used as the selector for selecting the parent ions for the daughter-ion spectrum. For much better results, an additional selector must be used, which will provide significantly better time resolution for the parent ions.
A prior art, optimized method for scanning daughter-ion spectra using time-of-flight mass spectrometers will appear as follows.
Ions are produced by the formation of a vapor cloud by laser bombardment focused onto a sample carrier of a solid sample containing analyte molecules embedded in a matrix substance. Due to the explosive expansion of the cloud in the vacuum, the ions have slightly different location-dependent, initial velocities. After a time delay, an acceleration field is switched on where, according to patent U.S. Pat. No. 5,654,545, for example, temporal focusing of the ions with a single mass but different initial speeds can be achieved at an adjustable location. The intermediate velocity is focused exactly on the location of the parent-ion separator, where the parent ions arrive, time-focused and at exactly the same time, so that high mass resolution can be achieved for the selection of a parent ion type. According to patent DE 196 38 577 C2 (U.S. Pat. No. 5,969,348), by using a temporal pulse profile for the acceleration field, it is possible to have the focal points for all the masses at the same location, i.e. in the parent-ion selector.
By means of a high energy density at the focal point of the laser, a high proportion of metastable ions ranging from a few tenths to a few percent can be achieved. In other words, these ions decompose to daughter ions and neutral particles as they travel through the mass spectrometer with a certain half-life. The daughter ions travel at the same speed as the parent ions. They are allowed through by the parent-ion separator at the same time as the parent ions but all other ions are masked out.
On leaving the parent-ion separator, the parent ions and daughter ions travel on to the potential lift and then drift slightly apart again due to small difference in the initial velocities. When they are in the potential lift cell, the potential is raised in a very short time. From here, they travel into the first acceleration path, which at this point has no field and is at the same potential. When transfer of the ions into the field-free acceleration path is complete, an acceleration potential is applied which, because of the location and velocity correlation of these ions, temporally focuses the ions of the same mass but different initial velocity as described in Patent Application DE 100 34 074.1. Final post acceleration is provided by a second acceleration path. The field strength of the first acceleration path is chosen so that, after passing through the ion reflector, the focal point lies precisely on the ion detector to achieve high mass resolution.
However, this relatively complex arrangement still has disadvantages. Metastable parent ions not only decompose before the potential lift but also after it. Although the rate of decomposition decreases exponentially with time, significant quantities of daughter ions are nevertheless produced in this region because the drift region up to the reflector is very long. The ions decomposing between the potential lift and the reflector produce slightly blurred peaks in the daughter ion spectrum. Although these can, in principle, be filtered out of the spectrum by calculation, they significantly reduce the quality of the spectrum.
The ions which are then produced by decomposition as they travel in the reflector are reflected without being focused and arrive at the ion detector at completely indeterminate times. They form a hump-type background noise extending over half the spectrum to the peak produced by non-decomposed parent ions. Because of the complete deceleration and re-acceleration of the ions, the decomposing parent ions remain in the reflector a long time. This is the reason for the significant amount of background noise.
Since only a small proportion of the parent ions in the mass spectrometer decompose into metastable ions before reaching the ion detector, the intensity of the peak produced by the parent ions is greater than that of all the other peaks by a factor between 10 and 10,000. This peak is therefore not only very high and saturates the electronic amplifiers but is also extremely wide since the feet of the peak are also raised. This creates an enormous amount of interference in the daughter ion spectrum and puts the secondary electron amplifier at risk, causing it to operate in a saturated state and thus producing fatigue phenomena which interfere with the spectrum scans which follows. On the other hand, this parent ion peak is the only peak of the daughter-ion spectrum of known mass and could be used for mass correction in the daughter-ion spectrum if its shape were suitable for accurate mass determination.
Precise mass determination from the flight times of the ions in a daughter-ion spectrum which has been obtained in this way is, however, exceptionally difficult. Because of the different switching times of the acceleration paths in the ion source and post accelerator, not only do the voltages used, which may be subject to drift, have an effect but, because of the digital time control system, effects on the flight time are produced due to rounding up to the smallest integer number for the time-control steps which amount to, for example, 5 or 10 nanoseconds. Due to this rounding error, parent ions of different masses are able to enter different distances into the post-acceleration stage and this has an effect on the total flight time. It would, therefore, be advantageous to have a reference mass in the daughter-ion spectrum.
A MALDI time-of-flight mass spectrum is not normally produced by a single laser shot. For a good mass spectrum, 20-200 laser shots are usually necessary. The digitized time-of-light spectra obtained from each laser shot are totaled to give a single time-of-flight spectrum, which is then converted to a mass spectrum by converting the times of flight. This is true both for normal mass spectra and daughter-ion spectra. When reference is made to the scanning of daughter-ion spectra in the following, this means exclusively daughter-ion, time-of-flight sum spectra, i.e. totalled ion-currents recorded against the times of flight.
The basic idea of the invention is to mask out, fully or partially, the parent ions and those daughter ions which are produced from the parent ions after post-acceleration due to metastable decomposition and which are responsible for the ghost peaks, from the post-accelerated ion beam by using a second ion selector. For masking to take place, there must be sufficient mass dispersion, i.e. a sufficiently long flight path must lie between post-acceleration and the second selector. For good mass resolution, the velocity focus of the post-acceleration path should be located in the second selector and the velocity-focusing reflector should image the velocity focal point onto the detector.
To obtain an optimum imaging of the velocity focus by the reflector onto the detector, however, it would be necessary to maintain a significant drift region between the intermediate velocity focal point and the reflector. With the two intermediate velocity focal points and the optimum imaging by the reflector, the entire mass spectrometer would then be extremely long and unmanageable. Fortunately, however, the mass resolution of the second ion selector does not need to be too high. The parent ions are the heaviest and slowest ions of the daughter-ion spectrum and form the rearguard of the group of ions in a daughter ion spectrum. Since there are no heavier ions, single-sided removal of the heavy parent ions is sufficient, which reduces the requirements in terms of mass resolution.
This type of mass spectrometry is mainly used in proteomics, in other words, one of the main uses of the instrument is found in the analysis of proteins and peptides. Here, it is the heaviest daughter ion which is of significant interest is the one with an amino acid split off. It is therefore at least approximately 70 atomic mass units lighter than the parent ions. This means that a high mass resolution and second velocity focus in the second selector is not necessary, so that a more conveniently sized instrument is possible.
However, by fully masking out the parent ions, the only peak which could be used as the mass reference is lost. Another idea of the invention therefore is to mask out only the overwhelming part of the parent ions or to introduce a parent-ion peak back into the daughter-ion sum spectrum but in such a way that practically no metastable ions are produced to give ghost peaks or background noise and any overdriving of the amplifier is avoided. Partial masking out can be performed by strong defocusing, e.g. by a lens. Full masking out can be achieved by a selector type in form of a multi-condensor like that of the parent ion selector. For the purpose of re-introducing a parent ion peak, a few individual spectra are acquired which only contain parent ions but with only a small ion beam intensity and without saturation. With MALDI spectra, the laser energy is regulated down by an attenuator so that practically no metastable ions are produced and, by switching off the masking system on the second selector, no saturated peak appears. By using only a few laser shots of this type, several separate spectra are acquired which contain practically only a single parent-ion peak. These spectra are added to the daughter-ion spectra in order to provide them with a parent-ion peak which is ideal as a reference for evaluation.
By using the parent ion peak as mass reference, the times of flight of the ions in the daughter-ion spectrum which have mainly been obtained from daughter ions by using non-attenuated laser shots and masking out the parent ions, can be converted into masses with outstanding results.